1. Field of the Invention
The present invention relates to an optical glass for an optical path and more particularly, it is concerned with an optical glass for an optical path, having a refractive index (nd) of 1.55 or more, Abbe number (.nu.d) of 48 or more and a specific gravity of 2.90 or less and being excellent in chemical durability.
2. Description of the Prior Art
As an element for general optical systems, there are used lenses, prisms, mirrors and the like. Optical glass has generally been used as a material for such an element requiring a high precision in many cases. This optical glass has been considered for use as a lens in view of its refractive index and dispersion. A lens can be regraded as an element for modification of light utilizing the refractive index and spherical surface of glass. Similarly, a prism is an element for modification of light. However, this is an optical element utilizing a pane of glass, not a spherical surface of glass, and in many cases, it reflects or transmits light into the interior part of the glass. It has thus been considered as an element for controlling the length of the optical path and for modifying the symmetry axis. Up to the present time, however, an optical glass developed and used for lenses has only been diverted to the purpose for controlling the length of the optical path, for example, a prism glass for a binocular, and there has been optical glass developed exclusively for a prism. When an optical glass for a lens is used for a prism, therefore, there are both merits and demerits. For example, optical glass commonly called "BK-7" and "BaK-4" have generally been used as a prism glass for binoculars. The properties of BK-7 and BaK-4 are shown in Table 1, in which the resistivity to acid is measured according to Japanese Optical Glass Industrial Standards "Measuring Method for Chemical Durability of Optical Glass (Powder method)".
TABLE 1 ______________________________________ BK-7 BaK-4 ______________________________________ nd 1.51633 1.56883 .GAMMA.d 64.0 56.0 Specific Gravity 2.52 3.11 Resistivity to Acid % 0.15 0.35 ______________________________________
BK-7and BaK-4 have generally the following composition (% by weight):
______________________________________ BK-7 SiO.sub.2 68.9 K.sub.2 O 8.4 B.sub.2 O.sub.3 10.1 BaO 2.8 Na.sub.2 O 8.8 As.sub.2 O.sub.3 1.0 BaK-4 SiO.sub.2 50.4 BaO 19.7 B.sub.2 O.sub.3 5.9 PbO 2.0 Na.sub.2 O 4.0 Sb.sub.2 O.sub.3 0.5 K.sub.2 O 5.0 As.sub.2 O.sub.3 1.0 ZnO 11.5 ______________________________________
As evident from Table 1, BK-7 has the advantages that the specific gravity is smaller and the chemical durability is relatively excellent, but has the disadvantage that the refractive index is smaller, while BaK-4 has the advantage that the refractive index is larger, but has the disadvantages that the chemical durability is lower and the specific gravity is larger. Since four prisms are used in one binocular and thus the weight thereof depends on the weight of the prism to a greater extent, the use of a prism with a light weight is of course important, but in the case of using a prism with a heavy weight, efforts to lighten the binocular have hitherto been made by using a light metal for the metallic parts of the binocular. However, a light metal tends to pose a danger of ignition when it is processed. It is thus preferable that the weight of a prism be as light as possible. Where a prism has a small refractive index, a binocular using the same has the disadvantage that the exit pupil as a conclusive factor of the performance of a binoclar is not of a true circle and there appear shadows (dark portions) at the four corners. Therefore, it is desirable that a binocular prism have a reasonably high refractive index of 1.55 or more is required for an ordinary binocular prism in order to remove such shadows. It is also desired that the Abbe number be as large as possible considering the chromatic aberration. In addition, the better the chemical durability of a prism is desirable. If the chemical durability is bad, tarnish takes place during polishing a prism, resulting in marked decrease of the yield of the prism, that is, increase of the production cost.
Moreover, optical glasses having a light weight and high refractive index have been proposed (Japanese Patent Publication Nos. 5127/1959 and 5128/1959), but they have been developed mainly as eyeglasses and thus they cannot avoid the disadvantages that in the case of using for optical paths, in particular, prisms, the chemical durability is insufficient, the Abbe number is too small and the percent transmission is inferior due to the length of the optical path.